Production of natural gas from shale has become an important energy supply in the U.S. However, shale strata vary widely in the capacity to be exploited for gas production. One factor which affects the potential for gas recovery is the pore architecture, e.g., pore connectivity, of a particular shale reservoir. Shales are sedimentary rocks which contain both organic and inorganic matter and porosity and wettability vary widely. An ability to evaluate porosity is important to an understanding of a shale reservoir and how it can be exploited.
Current technologies used to evaluate pore connectivity in shales are based on (1) mercury intrusion and (2) permeability measurements. For mercury intrusion, the most popular instruments are the Micrometrics Autopore™ series. The Autopore™ instruments inject mercury in increasing pressure steps to measures the pore volume accessible through pore throats of different sizes. The dimensions of the pore throats can be calculated from the mercury intrusion pressures. However, in shale samples, pressures greater than 5000 psi (e.g., 10,000 psi) are required for mercury to enter the pore space. The high pressures required for mercury intrusion compress the shales and reduce the size of compliant pores. Therefore the results obtained after mercury intrusion cannot be used in reservoir evaluation.
Several systems for permeability measurements are available. During permeability measurements, a fluid (generally gas) is flowed through a porous sample, and the pressure and/or flow rate data is analyzed to obtain a permeability value. The PDP 250™ instrument is used by Core Lab (a major provider for oil field services) to measure permeability. However, due to the fissile nature of shales, permeability measurements are often dominated by fractures and do not represent flow in the shale matrix.
Pore connectivity measures how the pore system is connected throughout a volume of rock. It is a physical concept which is often thought to be measured by permeability or effective porosity in conventional reservoirs rocks. Shale permeability measurements are often affected by the presence of fractures which act as bypass conduits, and measurement of effective porosity has been an elusive concept in shales. The standard methods used to evaluate pore connectivity in conventional rocks are not applicable in shales. Several authors have used the combination of focused ion beam (FIB) and scanning electron microscopy (SEM) to build 3D volumes of several shale samples. From the 3D shale volumes, they extracted the connected and non-connected pore systems and studied the connectivity levels in the samples. However, the extraction of the pore spaces from 3D volumes relies on the establishment of subjective gray scales thresholds for the pore systems; hence the resultant pore spaces will be strongly dependent on the researcher. Mercury injection capillary pressure (MICP) was used to study pore connectivity in shales. It was reported that mercury starts to intrude the shale samples at pressures greater than 5,000 psi. This translates into pore throat diameters smaller than 36 nm. The MICP experiment provides a pore throat size distribution, but does not allow the investigation of how the pores are connected. In order to study which pores are connected and how they are connected Wood's metal was injected into in shale samples. Wood's metal is an alloy that melts at 70° C. Molten Wood's metal was injected in the shale samples at a maximum pressure of 87,000 psi and 46,000 psi. The smallest pore throats accessible at 87,000 psi and 46,000 psi were, respectively, equal to 2.3 nm and 4 nm. After the injection of Wood's metal, the samples were cooled to room temperature while maintaining the maximum pressures. This process solidified the Wood's metal, and the distribution of Wood's metal throughout the samples was studied with the SEM. Wood's metal penetrated essentially the pores at the edge of the samples, microfractures, and the vicinity of the microfractures. The concentration of Wood's metal in the middle of the samples was quantified as equal to approximately 1/1000th of the concentration at the edges. Wood's metal and mercury are both non-wetting; hence a significant part of the mercury intrusion volumes recorded during MICP on shales did not enter the samples. The false intrusion reflects the compressibility of the shale sample.
Conventional methods fail to distinguish the different wettability systems contained in shale reservoirs. Therefore, new methods for determining pore connectivity and other pore characteristics of shale deposits are needed to improve the efficiency of shale gas production.